1. Field of the Invention
The present invention relates to a damper which damps impact forces applied to a member as an object for damping, such as a suspension system of a vehicle.
2. Description of the Related Art
Usually, a damper is suspended between a vehicle body side and a wheel side of a suspension system of a vehicle. Generally, the damper is provided with a cylinder tube connected to either one of the vehicle body side and the wheel side of the system, a free piston axially and slidably fitted in the cylinder tube to divide the inside of the cylinder tube into an oil chamber filled with oil and a gas chamber filled with gas, a piston axially and slidably fitted in the cylinder tube to divide the oil chamber into a first oil chamber and a second oil chamber, and a piston rod extending axially from the piston in a direction opposite to the gas chamber with the extending section being connected to the opposite side of the suspension system. Further, a small oil passage with a small cross section is provided in the piston.
When an impact force is applied externally to the suspension system, the damper is expanded and contracted, causing the oil to flow through the small oil passage between the first and the second oil chambers. Consequently, a damping force is generated, and the impact force is relaxed.
When the damper is expanded as described above, the piston rod is pulled outwardly. At this time, the capacity of the cylinder tube is decreased, and bubbles tend to occur in the oil present in the oil chamber. In other words, “cavitation,” which may give a negative effect to a damping force characteristic of the damper, tends to occur in the oil chamber. In this case, however, the free piston is pushed toward the oil chamber by gas pressure in the gas chamber. Thus, an occurrence of the cavitation is prevented.
Of both surfaces of the piston of the damper, one surface on a piston rod extension side has a pressure receiving surface area smaller than that of the other surface by a surface area corresponding to the connection of the piston rod with the piston. Accordingly, due to oil pressure generated in the oil chamber resulting from the gas pressure, the piston pushes and moves the piston rod outward of the cylinder tube. In other words, the damper starts to expand. Such an expansion of the damper always causes the vehicle body side and the wheel side to move away from each other and may deteriorate ride comfort of the vehicle. Therefore, such an expansion is not preferable.
To avoid such a drawback, conventionally, a damper of a through-rod type (a double rod type) provided with a pair of piston rods was disclosed in JP-A-Hei11-165521, for example.
According JP-A-Hei11-165521, the damper is installed between an axle arm and an end of a stabilizer of a vehicle. The damper includes a cylinder tube connected to the axle arm; a fixing rod guide for closing one end of the cylinder tube; another fixing rod guide fitted in the other end of the cylinder tube; a piston axially and slidably fitted in a middle section in the axial direction of the cylinder tube; a main piston rod extending on the piston side, passing through the one fixing rod guide, and connected to the end of the stabilizer; and a sub-piston rod extending on the piston side and passing through the other fixing rod guide.
A first oil chamber is defined between the one fixing rod guide and the piston. Further, there is also a second oil chamber defined between the other fixing rod guide and the piston. Small oil passages with a small cross section arranged to connect the first and the second oil chambers are formed in the piston and the piston rod. In addition, there is provided a gas reservoir which reserves high-pressure gas and normally pressurizes the oil in the first and the second oil chambers via a free piston displaced by gas pressure thereof.
For example, when an impact force is applied to the damper via the axle arm, the cylinder tube and respective piston rods relatively move. Accordingly, when the damper expands or contracts, the oil flows through the small oil passage between the first and the second oil chambers. Consequently, damping force is generated, and the impact force is damped. As a result, a tendency to generate vibrations between the axle arm and the stabilizer is decelerated.
The main piston rod and the sub-piston rod have the same diameter. Accordingly, the pressure receiving surface areas on both sides of the piston are kept equal to each other. Consequently, it is prevented that the piston rod is, as described above, pushed out of the cylinder tube because of gas pressure. Further, while the damper is operated, the total capacity of the first and the second oil chamber does not vary. Therefore, it is prevented that the oil pressure is unnecessarily increased according to the operation described above. On the other hand, if the volume of the oil varies due to a temperature change, such variance in the volume is smoothly mitigated by a volume change resulting from compressibility of the gas. As a result, it is further prevented that the oil pressure is unnecessarily increased.
In addition, the oil is normally pressurized by the gas pressure. Accordingly, when the damper is operated, or when the volume of the oil varies according to a temperature change, it is prevented that cavitation tends to occur in the first and the second oil chambers. Consequently, the damping force characteristic of the damper is kept excellent.
Further, the damper is provided with a regulating valve which makes it possible to adjust the opening of the small oil passage provided on the axial center of the sub-piston rod and an operating rod fitted in an axial center hole formed on the axial center of the main piston rod, having one end connected to the regulating valve, and having the other end as an operating section operable outside of the one end of the cylinder tube.
In conjunction with operation of the operating section, the regulating valve is operated via the operating rod to adjust the opening of the small oil passage. Because of this adjustment, the damping force characteristic of the damper becomes variable.
However, the damper has a problem, which will be described herein after.
Firstly, the damper is provided with the gas reservoir having the free piston separately from the cylinder. Therefore, there are a large number of parts. Accordingly, the construction of the damper is complicated. Moreover, since the cylinder tube and the gas reservoir are disposed in parallel, the whole shape of the damper tends to become larger.
Secondly, a connecting section is provided on the other end of the cylinder tube to connect the cylinder tube and the axle arm. Moreover, the cylinder tube containing a trajectory of the sub-piston rod at a time when the damper is contracted and the connecting section are provided in parallel in the axial direction of the damper. Accordingly, the total length of the damper tends to be long.
On the other hand, extra space available in a vehicle is narrow in general. Accordingly, if the damper is provided in such a space, it is difficult to make the total length thereof sufficiently long. Thus, a piston stroke during operation is restricted to be short. As a result, the damper may not sufficiently damp impact forces when the vehicle passes a very rough portion on the road.
Thirdly, the damper has the operating rod fitted in the axial center hole formed on the axial center of the main piston rod as described above. Further, the operating section of the operating rod is operable from the outside area of the one end of the cylinder tube.
However, it is possible that an operation needs to be performed from an outside area of the other end of the cylinder tube depending on a mounting condition of the damper. Notwithstanding this, the connecting section is formed on the other end of the cylinder tube as described above. Consequently, such operation from the outside area of the other end of the cylinder tube as described above cannot be performed because the connecting section is obstructed. In other words, there is a problem in which the degree of freedom is low in relation to an operation of the damper for obtaining a desired damping force characteristic.